These studies are designed to ascertain the effect of aging on hypothalamic-hypophyseal secretion of the antidiuretic hormone, arginine vasopressin (AVP), in response to: 1) overnight dehydration, 2) hypertonic saline infusion and 3) intravenous ethanol infusion. These standardized tests of both negative (ethanol) and positive (hypertonic saline) stimuli to the hypothalamic-hypophyseal axis showed: (1) with aging there is an enhanced sensitivity of AVP secretion to hyperosmolality; (2) the inhibitory effects of ethanol on AVP do not change with age during the first 30 minutes of the infusion; then there is a paradoxical loss of the inhibition in older subjects; (3) the transient effect of ethanol in the aged is probably due to the increase in serum osmolality induced by the ethanol, thus secondarily causing a counter-effect of AVP secretion. The increased sensitivity of this endocrine mechanism with age is probably an important factor in the known susceptibility of older patients to the development of serious hyperosmolality syndromes.